Most of the troubles stem from water and fertilizing issues. But beware the hornworm.
Just as your first tomatoes set and begin to swell, something collapses the bottom of the tomato, turning it to brown or black leather, or mush. The fruit is ruined. Go ahead and pull it off the vine. You can compost it, since a disease doesn't cause it. It's a weather- and gardener-related problem called blossom end rot. And there's an easy fix.
Tomatoes need calcium. A steady source of moisture helps them take it up. They need consistent water when mature, a quart a day, to ward off blossom end rot. If your tomatoes have succumbed to this, you can purchase and drench your plants with a calcium mixture made just for the problem. Or simply keep the plants watered, which will allow the vines to take up calcium again, and they should cure themselves.
Put mulch around the plants to hold moisture around the roots. And before you plant your next crop, check your soil's pH level and adjust it to between 6.0 and 7.0. Your local garden center can tell you what you need.
Big ugly green worms are also a common problem on tomato plants. Evidence of the pest may show up first as leaves eaten down to their ribs. On closer inspection, you may notice loose black dots (excrement) on your plants. Study the tops and bottoms of the leaves and stems and soon you will spot the culprit, a large hornworm.
There are two types of hornworms infesting gardens in the United States. Tomato and tobacco hornworms look similar but differ slightly in their coloring: The tomato hornworm sports a black-sided dark green "horn" on its backside; the tobacco hornworm has a red "horn." The white striping on their sides is also different, but it really doesn't matter which hornworm is eating your tomato plants. Either one can be found in your garden, and both can defoliate a tomato plant in days.
To control the pests, take a pail of soapy water to the garden, pull off the caterpillars, and drop them in the water. (If you are squeamish, wear gloves.) Mature hornworms are often quite large, 3-1/2 to 4 inches long. You have nothing to fear from the "horn" they raise as a defensive ploy. It is only for show.
A good natural predator for hornworms is a female braconid wasp. These tiny (an eighth of an inch), well-behaved wasps won't bother you but can kill off hornworms, which serve as hosts for their eggs.
If you spot small white cocoons hitching a ride on a hornworm in your garden, you'll know help is on the way.
A number of fertilizer deficiencies as well as diseases can cause leaf yellowing. Keep your tomato plants fertilized throughout the growing season. They require nitrogen, phosphorus, and potassium for steady growth. You can prevent leaf yellowing at the base of the plant to some extent by removing low leaves when you put your plants in the ground. Keep the leaves pruned off the ground on staked plants to keep waterborne diseases from splashing onto the leaves. Look for tomato plants or seeds with the letters F.V.T.N., which means they are resistant to many diseases.
Heavy rain can cause small skin splits on ripe fruit. That same heavy rain, coupled with high temperatures, can cause cracking at the stem and around the shoulders of the fruit. Although the fruit might not look perfect, the taste is not affected unless rot sets in.
Poor pollination can cause blossoms to drop before they set fruit. Do not use lingering insecticides on vegetable flowers, and preferably none at all. You'll end up killing the bees and other pollinators as well. A stressed plant might also drop its flowers. This can occur with low moisture and high temperatures.
A warm tomato fresh from the vine in one hand and a saltshaker in the other make sweet memories. Tomatoes are the most popular and easy-to-grow backyard crop. To ward off most problems, just keep them watered and fertilized.